1. Technical Field
The present invention relates to a pulse width modulation (PWM) drive control of a multi-phase electric motor such as a three-phase brushless motor. In particular, the present invention relates to a noise prevention technique of a controller of a multi-phase electric motor arranged with a single current detector between a drive circuit for PWM driving and a direct current (DC) power supply (high voltage side or low voltage side).
2. Related Art
In a controller for driving a multi-phase electric motor such as a three-phase brushless motor, a PWM signal for determining ON/OFF timing of a switching element for driving the multi-phase electric motor is generated by comparing a carrier wave of saw-tooth shape or triangular shape (saw-tooth signal, triangular signal) and a duty set value corresponding to a target current value in each phase of the multi-phase electric motor. That is, whether the PWM signal is high level or low level is determined depending on whether a value (value of PWM counter) of the saw-tooth signal or the triangular signal is greater than or equal to, or smaller than a duty set value.
The time interval in time of switching between one phase and another phase sometimes becomes very small in the controller of the multi-phase electric motor for generating the PWM signal based on the saw-tooth signal and the triangular signal, and driving the multi-phase electric motor. In this time, since the current is not stable due to the switching time of an electric field effect transistor of the drive circuit, the presence of dead zone (dead time), and also the response delay of an electronic processing circuit, the measurement of an accurate current value by a current detector cannot be carried out during such a period.
For instance, when using an A/D converter for the current detector, an accurate current value cannot be detected unless a stable signal is continuously inputted for at least 2 μs according to the specification of the A/D converter. If the input signal is not stably inputted continuously for 2 μs, the A/D converter cannot detect an accurate current value of each phase.
In a vehicle steering device described in Japanese Unexamined Patent Publication No. 2007-112416, a single current sensor for detecting the current value flowing through a current path is arranged on the current path between a motor drive circuit and a ground, and a phase of a saw-tooth wave for generating the PWM signal of each phase is shifted to shift the timing of fall of the PWM signal of each phase to the low level. A value of a U-phase current flowing through the electric motor is then obtained based on an output signal of the current sensor during a period in which a predetermined time has elapsed from when the PWM signal of a V phase fell to the low level. A total current value of the U-phase current and a V-phase current flowing through the electric motor is obtained based on an output signal of the current sensor during a period in which a predetermined time has elapsed from when the PWM signal of a W phase fell to the low level.
In a method of controlling a three-phase or multi-phase inverter described in Japanese Unexamined Patent Publication No. 10-155278, if the time interval between the switching of a transistor of one phase and the switching of a corresponding transistor of the next phase is smaller than a predetermined threshold value within a PWM period, the measurement is prohibited, the PWM signal defining the measurement time interval of sufficient duration is generated, and the influence of switching on a line current can be measured. The duration of the other PWM signals of the same dependent period is reduced by a certain value, and the sum of reduction of such other PWM signals is obtained to compensate for the amount of increase of the PWM signal defining the measurement interval.
A drive system for a three-phase brushless AC motor described in Japanese Unexamined Patent Publication No. 2005-531270 is configured to optimize a transistor switching pattern in order to enhance the power output while enabling the measurement of the current in all phases using a single sensor. This is realized by defining a voltage demand vector x in a case where three or more states are required to satisfy a minimum state time requirement determined by the single sensor method, and calculating the three or more state vectors for generating the request vector x while still allowing the single current detection.
In a method of monitoring a brushless motor capable of compensating some kind of drift in an output signal during a motor operation described in Japanese Unexamined Patent Publication No. 2001-95279, the current flowing into or flowing out from each winding of the motor is monitored and an output signal displaying the current is generated using a current measurement section, the output of the current measurement section is measured when an instantaneous current flowing through the current measurement section is known to be substantially zero, and a correction output signal for compensating some kind of difference between an actual measurement output signal value and an ideal output signal value is generated.
In U.S. Pat. No. 6,735,537, a triangular signal is used for a carrier wave, terms h phase, m phase, and l phase are used in place of the terms U phase, V phase, and W phase, where the time interval between the h phase and the m phase is represented as t1 and the time interval between the m phase and the l phase is represented as t2. As shown in FIG. 7 of U.S. Pat. No. 6,735,537, the process of Case 2 is performed when the time intervals t1, t2 are both smaller than a threshold value (mw). The process of Case 3 or Case 4 is performed when either one of the time intervals t1, t2 is smaller than the threshold value (mw). In the case of the process of Case 2 (see FIG. 13), the Duty maximum phase is shifted to the left side, and the Duty minimum phase is shifted to the right side (see FIG. 12B). If in the case of the process of Case 3 (see FIG. 15), and determined that only one phase needs to be shifted (N of step 148), the Duty maximum phase is shifted to the left side (see FIG. 14B). If in the case of the process of Case 4 (see FIG. 17), and determined that only one phase needs to be shifted (N of step 166), the Duty minimum phase is shifted to the left side (see FIG. 16B).
When the time interval in time of switching between one phase and another phase is small, the time interval in time of switching between one phase and another phase becomes large by performing a correction of shifting the phase of a predetermined phase, and an accurate current value of each phase of the multi-phase electric motor can be detected using the single current detector. However, if the frequency of the ON/OFF of the switching element for driving the multi-phase electric motor is included in an audible frequency as a result of performing the shift correction, it is heard by the user as noise and gives the user an unpleasant feeling.
For instance, in the control method of Japanese Unexamined Patent Publication No. 10-155278, a control frequency and the corrected current ripple frequency are the same when the PWM signal is corrected. In the control method of Japanese Unexamined Patent Publication No. 10-155278, a control cycle time (period) is 400 μs, and thus the control frequency and the corrected current ripple frequency become 2.5 kHz. The current ripple is generated in time of switching by turning ON/OFF the switching element based on the corrected PWM signal. If the frequency of the current ripple is included in the audible region, it is heard by the user as noise and gives the user an unpleasant feeling. Humans are able to feel the sound normally from about 20 Hz to 15 kHz or from about 20 Hz to 20 kHz, which differs among individuals, and such frequency band is referred to as the audible region. That is, the noise is generated when having the control cycle time of between 50 μs and 50 ms. The following techniques are proposed to prevent such noise.
A motor drive device of an electric power steering described in Japanese Patent No. 2540140 assumes one switching element of each pair for conduction holding and the other switching element for high-speed switching of the switching elements of two pairs, and has the frequency of a pulse width modulation signal for high-speed switching higher than an audible frequency region, and thus the linearity of the output torque of the motor with respect to a steering torque can be enhanced by effectively utilizing a current continuation effect by a flywheel diode, and the generation of vibration sound can be prevented regardless of the switching by the pulse width modulation signal.
An inverter device described in Japanese Unexamined Patent Publication No. 63-73898 generates the PWM signal by comparing a modulation wave signal obtained by amplifying an error of a magnetic flux command signal of the frequency proportional to the frequency command from the outside and a motor voltage integration signal outputted by an integration circuit for integrating an inverter output voltage, and a triangular signal which is the carrier frequency of the non-audible frequency.
A controller of an electric vehicle described in Japanese Unexamined Patent Publication No. 9-191508 drives a motor with the power of a battery by PWM controlling an inverter arranged between a battery and a motor with a PWM control section, and normally sets the frequency of the PWM control section higher than an audible frequency to reduce the switching noise of the inverter. When a motor operation state detection section detects that the motor is in a low-speed, high-load operation state, and there is a possibility the switching element of the inverter may overheat, a frequency changing section lowers the frequency of the PWM control section to prevent damage by overheat of the switching element of the inverter.
However, there is not yet proposed a controller of a multi-phase electric motor capable of generating the PWM signal based on the saw-tooth signal or the triangular signal, and detecting the current value of each phase at satisfactory precision for every control period using a signal current detection section, and having a sufficient noise prevention effect.
FIG. 8 shows a diagram showing a comparison example not dependent on the present invention, and is a timing chart in a case where two phases are not detectable. One control period is 250 μsec, and includes five periods of the PWM signal based on the saw-tooth signal of 50 μsec period. In the figure, an operation in the fourth and the fifth periods of the previous control period T1, and the first to the fifth periods of the present controller period T2 is shown. In the previous control period T1, a case where the PWM signal of A phase is duty 52%, the PWM signal of B phase is duty 47%, and the PWM signal of C phase is duty 51% is shown. Since the time interval between the B phase, which is the duty minimum phase, and the C phase, which is the duty intermediate phase, and between the C phase, which is the duty intermediate phase, and the A phase, which is the duty maximum phase, is 4% and 1%, that is, short respectively, the switching noise of the relevant period cannot be accommodated unless the phase is shifted, and the A/D conversion time for accurately detecting the current value cannot be ensured. Thus, the phase of the PWM signal of the B phase which is the duty minimum phase is shifted to the left side (to advance phase) by 8%, and the phase of the PWM signal of the A phase which is the duty maximum phase is shifted to the right side (to delay phase) by 11%. Thus, both switching time intervals between the B phase and the C phase, and between the A phase and the C phase become 12%, that is, large, and the accurate current value of the A phase and the B phase can be detected in each PWM period.
An operation in the first to the fifth periods of the present control period T2 will now be described. In the present control period T2, the PWM signal of the A phase reduces from the duty 52% to 51%, the PWM signal of the B phase does not change at the duty 47%, and the PWM signal of the C phase increases from the duty 51% to 52%. Therefore, the duty maximum phase changes from the A phase to the C phase, and the duty intermediate phase changes from the C phase to the A phase. The duty minimum phase is again the B phase. Since the time intervals between the B phase, which is the duty minimum phase, and the A phase, which is the duty intermediate phase, and between the A phase, which is the duty intermediate phase, and the C phase, which is the duty maximum phase, are 4% and 1%, that is, short respectively, the switching noise of the relevant period cannot be accommodated unless the phase is shifted, and the A/D conversion time for accurately detecting the current value cannot be ensured. Thus, the phase is shifted to the left side (to advance phase) by 8% for the PWM signal of the B phase which is the duty minimum phase, the phase is shifted to the right side (to delay phase) by 11% for the PWM signal of the C phase which is the duty maximum phase, and the PWM signal of the A phase which is the duty intermediate phase is not shifted.
Thus, in each of the five PWM periods of the present control period T2, both switching time intervals of between the A phase and the B phase, and between the C phase and the A phase become 12%, that is, large and the accurate current value of the A phase and the B phase can be detected in each PWM period. With respect to the timing of performing the A/D conversion, in any period, the detection of the current value of the B phase is performed in a period necessary for the A/D conversion immediately before the fall of the PWM signal of the A phase which is the duty intermediate phase in an even number vector state (1, 0, 1), and the detection of the current value of the C phase is performed in a period necessary for the A/D conversion immediately before the fall of the PWM signal of the C phase which is the duty maximum phase in an odd number vector state (0, 0, 1). The vector will be hereinafter described in the section of the description of one or more embodiments of the present invention.
This example is a case where change is made from shift to no shift for the A phase, shift is made but the shift amount is not changed for the B phase, and change is made from no shift to shift for the C phase. Thus, when shift/no shift changes due to the change in the magnitude relation of the duty of each phase in the previous and the present control periods T1, T2, an instantaneous current fluctuation produces as shown in the shunt waveform (waveform of the voltage generated over both ends of a current detection shunt resistor) at an end time of the previous control period T1, that is, at a start time of the present control period T2. With the sudden current fluctuation, noise based on the current ripple generates from the motor. The shunt waveform shows the current of the A phase and the −B phase in the previous control period T1, and the current of the C phase and the −B phase in the present control period T2. The waveforms are different.